1. Field of the Invention
The present invention relates to an organic electroluminescent device, more particularly, to an organic electroluminescent device capable of being driven at low voltage.
2. Description of the Related Art
An organic electroluminescent device (OELD) has advantages in that it is thin and lightweight. The components and processing are simple, high definition and a wide view angle are secured, a complete moving picture and high color purity are realized, and electrical characteristics appropriate a for mobile purpose are obtained by low power consumption and low voltage driving of a self-emitting type display using a phenomenon that light is generated as electrons are being connected to holes in an organic compound layer when current flows to a fluorescent or a phosphorescent organic compound thin film.
The organic electroluminescent device is divided into a passive driving device and an active driving device according to a driving method, a small molecular weight device and a polymeric device according to the material and the structure.
In the case of a small molecular weight device, the efficiency and lifetime are improved by forming a hole transport layer between the emitting layer and the anode and forming an electron transport layer between the emitting layer and the cathode, with the emitting layer being substantially between the anode and the cathode as mentioned in U.S. Pat. Nos. 4,356,429, 4,539,507, 4,720,432 and 4,769,292. These patents substantially include color patterning by deposition and technologies commonly applied to passive and active types of display devices.
On the other hand, an initial polymeric device, different from the small molecular weight device, is formed in a sandwich shaped structure, wherein a light emitting polymer is introduced between a transparent anode and a metal cathode as mentioned in U.S. Pat. No. 5,247,190. The polymeric device has disadvantages in that the efficiency and the lifetime are unfavorable since the consistency between the work function of both electrodes and the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) values of polymers are not well matched. On the contrary, the efficiency and the lifetime are quite improved by introducing a metal for the cathode having a work function value similar to the LUMO value of the polymer at the cathode side. It is difficult to introduce material except ITO and IZO since there is a limitation of a transparent electrode at the anode side.
A structure to adopt a single layer comprising a light emitting polymer between the anode and the cathode is typical due to the advantages that the functions of the electron transport and the hole transport of a general light emitting polymer are simultaneously performed when fabricating a full color PLED device. However, a four-layer structure adopting a first charge carrier injection layer, two layers of a semiconductor conjugated polymer and a second charge carrier injection layer is adopted in U.S. Pat. No. 5,807,627, wherein the efficiency of the device is increased by adding charge carrier injection layers even though implementation of the improved electron transport and the hole transport is difficult. However, there is a limitation wherein polymers having different characteristics should be adopted in a single device so that a lower layer is not dissolved during wet processing, and only mono color is realized because an emitting layer may not be patterned since the total process proceeds by spin coating in the case of adopting PPV series polymer adopting a multilayer structure, thus constructing two layers.
Eventually, the necessity to increase the efficiency or the device stability gain force even in a polymeric light emitting device by eliminating a simple structure has been recognized, and the high molecular weight device also requires addition of a subsidiary layer to aid injection or movement of the charge carriers as in the lower molecular weight device.
Each layer may be introduced by vacuum deposition in the case of a small molecular weight organic electroluminescent device. However, introduction of various organic layers is restricted since the spin coating process is used in the case of the polymeric organic electroluminescent device. Therefore, the polymeric organic electroluminescent device is generally constructed in a simple structure having anode and cathode electrodes, a hole injection layer and an emitting layer.
However, the polymeric organic electroluminescent device not only has problems due to diffusion of the anode material indium or tin into the emitting layer, or because of a change in the emitting layer due to use of a hole injection layer having acidic characteristics, since the device is fabricated without a separate hole transport layer or an electron transport layer in the upper and the lower parts of the emitting layer due to its single layer structure. This also causes a negative effect in that the stability of the device is lessened considerably due to diffusion of the cathode metal into the emitting layer that may occur when depositing the cathode material.
One of the methods to solve the above-cited problems is a method to prevent diffusion by forming a thick hole-blocking layer (or an electron transport layer) or a hole transport layer in the upper and the lower parts of the emitting layer. However, the hole transport layer, the hole-blocking layer and the electron transport layer contain a large amount of self-emitting materials and change the wavelength of the light coming out of the emitting layer by forming an exciplex on the interface with the emitting layer, even through the materials do not emit light by themselves. Furthermore, there is a problem that forming thick materials in the upper and the lower parts of the emitting layers considerably increases the driving voltage.
On the other hand, a combined concept of reduction of the driving voltage and the luminance quenching principle by promoting a non-radioactive process different from emission does not exist, although existing patents on doping mainly concern the concept of energy transfer using a method for mixing the emitting dopant with the emitting host substances (U.S. Pat. No. 4,769,292) or a concept of preventing trapping of the electrons by simply mixing materials having low LUMO value with a hole transport layer or an emitting layer (U.S. Pat. No. 5,601,903). This concept is also adopted to the small molecular weight (or hybrid type) organic electroluminescent device as well as the polymer light emitting device.